Electronic ballast is widely used because of its advantages of high efficiency, energy saving and compact size. However, it is still not as popular as the conventional magnetic ballast. This is because electronic ballasts are often compared directly with magnetic ballast, both in terms of performance and cost. An electronic ballast has to meet many regulations for lighting apparatus such as those for input harmonic current, power factor, total harmonic distortion. Very often high-performance and expensive components are required in order to meet these regulations. For example, high voltage electrolytic bulk capacitor are usually needed in a ballast circuit, but the life time of most high voltage electrolytic capacitor is 2,000 hours at rated condition, which is only half the life time of a tube type fluorescent lamp. So there is very tough trade off between cost and reliability of an electronic ballast.
A typical prior art ballast circuit is shown in FIG. 1. It consists of a rectifier, a boost converter followed by a DC to AC converter. The rectifier converts the AC input to a pulsating DC source. The boost converter serves as a Power Factor Correction (PFC) front end which make sure that the input current meet the regulatory requirements. The DC to AC converter receives the DC from the PFC front end and produces a plurality of pulses by switches M1 and M2. The pulses are coupled to a resonant circuit which consists of a lamp load. When the pulse frequency is close to the resonate frequency of the resonate network, a lot of power will be delivered to the load. If pulse frequency is slightly shifted with respect to the resonate frequency of the resonate network, power delivery will drop. The deviation of power caused by frequency shift depends on the Q factor of the resonate network. Also the maximum current flow into the lamp depends on the series inductance Lres and the lamp characteristic. The major drawback of this prior art is the sensitivity to component variations because resonant is key of the operation. The operating point must fall into a high gain region of the resonant characteristics otherwise the lamp would not light up properly.
When lamp dimming is needed an external phase control dimmer is often used. This calls for more complicated circuits in the ballast. Work of this type can be found from U.S. Pat. No. 5,172,034 by Brinkerhoff, U.S. Pat. No. 5,396,155 by Bezdon et al, U.S. Pat. No. 5,559,395 Venkitasubrahmanian et al, U.S. Pat. No. 6,094,017 by Adamson, U.S. Pat. No. 6,339,298 by Chen, U.S. Pat. No. 5,686,799 by Moisin, U.S. Pat. No. 5,825,137 by Titus, U.S. Pat. No. 6,100,644 by Titus, etc. The basic circuit is similar to the prior art with a power factor corrector front end in cascade with a converter to produce a pulsating voltage to a resonate circuit. Basically the idea is to generate a control signal to shift the pulse frequency along the bell shape resonate characteristic curve of the resonant circuit in order to adjust the power delivery produces dimming effect on the lamp. The control signal can be provided by an external controlling device, a potentiometer, or the average phase conduction angle voltage of an external dimmer. This type of control method cannot be very stable because the resonant circuit characteristics is very sensitive and changeable.
Some researchers attempted to solve the stability problem of dimmable ballast. Work in this area can be found from U.S. Pat. No. 5,315,214 by Lesea, U.S. Pat. No. 6,037,722 by Moisin, U.S. Pat. No. 6,118,228 by Pál, U.S. Pat. No. 6,144,169 by Janczak, U.S. Pat. No. 6,448,713 by Farkas et al, U.S. Pat. No. 6,452,344 by MacAdam et al try to sense the current lamp current and compare it with the control signal using feedback control and adjust the switching frequency to go to a stable operating point on the bell shaped resonant curve. Many complex circuits are needed, together with the power factor corrector front end the final product is not cost competitive.
Some other researchers try to use simper circuits to achieve both good power factor and dimmable effect. In U.S. Pat. No. 5,801,492 Bobel uses a single stage circuit to provide power factor correction but it requires two resonant circuits to allow energy to flow back to the rectified input side and cause high voltage stresses on the main switches. In U.S. Pat. No. 6,348,767 Chen et al use two resonate circuit and connect the lamp loading to input side to provide a small continuous current flow to hold the triac dimmer on the input side but it produces poor power factor. In U.S. Pat. No. 6,011,357 Gradzki et al use a separate circuit to keep a small continuous current flow to hold the triac dimmer on the input side with poor power factor. In U.S. Pat. No. 6,429,604 B2 Chang uses multiple LLC resonant circuit to control the input current shape and lamp current flow but voltage stress is higher than the input peak AC voltage. This produces excessive voltage stresses on the components in the circuit.
There is a need to develop a ballast to have a simple circuit, stable operation, low input current harmonic characteristic and low electrical stresses.